Fire is a self-sustained fuel oxidation process that produces changes in the surrounding environment such as:                Temperature increases,        Concentration of various gases changes, particularly O2, CO2, CO and H2O        Flames occur in some fires        Smoke is generated in many fires        Physical properties such as viscosity, speed of sound change due to temperature increase and changes in gas concentration        
Fire detection devices rarely go into alarm, but even when they do it is at times the case that alarm is not due to a fire. For example, dust can be mistaken for a fire-produced smoke and alarm is generated. There is a need to minimize number nuisance alarms like that one while maintaining or improving speed of response to a real fire.
Successful discrimination between fires and nuisances depends on the ability to sense different characteristics of fires in cost-efficient way. Signal processing from multiple sensors minimizes the probability of generating an alarm due to a nuisance stimulus while increasing speed of response to a real fire.
Choice of a sensing element, or elements, depends on many factors. Sensors should preferably be responsive to many if not all types of fire. A sensor should also be reliable, rugged, small, and inexpensive, with a good signal-to-noise ratio while consuming small amounts of electrical power.